1. Field of the Invention
The present invention relates to a surface acoustic wave device for use as a bandpass filter, a resonator, or the like, and more particularly relates to a surface acoustic wave device including at least one IDT electrode and a piezoelectric film composed of ZnO, which are laminated on a piezoelectric substrate composed of quartz.
2. Description of the Related Art
Heretofore, various surface acoustic wave devices have been proposed using various piezoelectric materials. For example, in “Small and Low-loss Filter Using ZnO Film/Quartz Structure” 31th EM Symposium, pp. 87 to 94 (May 2002), a small surface acoustic wave filter having the structure in which a ZnO film and quartz are laminated to each other has been disclosed. In this document, a transversal type surface acoustic wave filter has been disclosed in which at least one IDT electrode composed of Al is formed on a piezoelectric substrate composed of 27° Y-cut X-propagation quartz, and in which a piezoelectric film composed of ZnO is laminated so as to cover the IDT electrode. A thickness H/λ of the IDT electrode normalized by the wavelength of a surface acoustic wave is set to 0.02. A normalized thickness H/λ of the piezoelectric film composed of ZnO is set to 0.3, and accordingly, a 380-MHz bandpass filter in the IF stage of WCDMA mobile phones is formed.
As disclosed in the above mentioned document, heretofore, a laminate structure of ZnO/IDT electrode/quartz has been studied for application to a transversal surface acoustic wave filter device.
In the meantime, as the surface acoustic wave device, besides a transversal surface acoustic wave filter, a surface acoustic wave resonator using reflection of a surface acoustic wave at an electrode and a resonator type surface acoustic wave device, such as a surface acoustic wave resonator type filter, have been known. However, the application of the structure of ZnO/IDT electrode/quartz to a resonator type surface acoustic wave filter device has not been sufficiently studied.
In recent years, as a bandpass filter of a mobile phone, a bandpass filter has been desired which has a narrow bandwidth with a fractional bandwidth of approximately 1% and has superior selectivity. For example, in video broadcasting service for mobile phones, a bandpass filter has been desired which has a center frequency of 700 MHz and a bandwidth of 6 MHz (fractional bandwidth of 0.85%). It has been very difficult for a transversal surface acoustic wave filter to realize a bandpass filter having a narrow bandwidth and superior selectivity as described above. In addition, even by a longitudinal coupling resonator type surface acoustic wave filter using LiTaO3, LiNbO3, or quartz as a piezoelectric substrate, the bandwidth as described above has been difficult to be realized.
In a resonator type surface acoustic wave device, in the case in which a wavelength determined by the pitch of an IDT electrode is represented by λ, a phase velocity on a piezoelectric substrate when an IDT portion is in an electrical open state is represented by V, V-Vm is represented by ΔV in which Vm indicates a phase velocity when the IDT portion is in a short-circuit state, a center frequency is represented by F, and a bandwidth is represented by ΔF, an electromechanical coupling coefficient K2 satisfies the following equation (1).K2/2=|ΔV|/V=|ΔV/λ/(V/λ)α|ΔF|/F  Equation (1)
When the resonator type surface acoustic wave filter device is miniaturized while the above fractional bandwidth of 0.85% is maintained, it is necessary to increase a reflection coefficient of electrode fingers of the IDT electrode. However, heretofore, in the resonator type surface acoustic wave filter device, it has been difficult to sufficiently increase the reflection coefficient, and hence a resonator type surface acoustic wave filter which can satisfy the demands as described above has not been realized.